Yafaray Export Button doesn't show in Blender Render Menu
You'll need to change render engines. I'm not sure that the Yafaray plugin exports the XML (I don't have Blender open now so I can't check). The same dropdown you use to change the render engine from Blender Internal to Cycles is the one you use to change to Yafaray. However, you'll need to enable the Yafaray plugin (Preferences, Addons).